The present invention generally relates to the field of emergency back-up power supplies, and more particularly is directed to a stand-by uninterruptible power supply having a current waveshape circuit for rating the power capacity of the supply.
Stand-by uninterruptible power supplies (UPSs)and other forms of backup, standby and emergency power systems are well known in the prior art. Such power supplies are used to provide power to various loads when the commercial AC power supplied by the utility company is interrupted momentarily due to power failure or scheduled maintenance.
The majority of applications for stand-by UPSs are for data processing electronic devices such as computer devices which have a power range of 3 KVA and lower. In order to save on the cost of the inverter in the UPS, in some applications there is widespread use of square waveforms as illustrated in FIG. 1 and step waveforms as illustrated in FIG. 2.
These applications are usually limited to a power range somewhat below 500VA as sharp excursions of voltage and voltage harmonics cause malfunctions in numerous loads. The reason for this is that the primary load in power ranges below 500VA is a personal computer or monitor. Both use switching mode power supplies which react only on voltage peaks and not on harmonic content. Nonetheless, a sharp rate of rise or fall of voltage on edges of the step wave or the square wave causes some amount of distortion in video monitors. Another disadvantage of UPSs which produce the waveforms illustrated in FIGS. 1 and 2 is that they cause output current having a high peak value.
A typical circuit involving a prior art UPS output circuit and the load power supply input circuit is shown in FIG. 3. FIG. 4 illustrates the voltage and current diagrams showing high peak current produced by this circuit.
A current peak is limited only by the resistance of the power semiconductors and wires. These peaks are high and require larger size and higher cost inverter semiconductor switches. These peaks also cause overheating of components in the load as the heating effect is proportional to I.sup.2 (square of the current).
For this reason, square or step waveforms are used only when the duration of the load operation is limited to approximately an hour. Otherwise, a sine waveform is used.
Other problems associated with prior art UPSs include confusion over their power rating capacities. This problem has become of concern as UPS usage has become more wide spread. Such confusion is of particular concern with respect to lay consumers who usually do not have sufficient technical skills to read and understand power supply specifications. The problem is made more severe by manufacturers who often strive to separate their product from competitors by using confusing and very technical terminology to describe product features and benefits. Thus, there is often no effective way for a consumer to compare competing products. This is especially so with respect to power capacity rating as manufactures usually express such ratings using units of measure which portray their product in the best light. Thus, the stated power rating of one piece of equipment may not be easily compared to the stated rating of another.
Accordingly, there is a need in the art for a UPS which overcomes the above noted deficiencies with respect to peak current, thereby allowing for either extended backup time and/or increase in power rating for UPS applications. Moreover, there also is a need for a more effective way of rating the capacity of an uninterruptible power supply so that different supplies may be easily compared with little confusion.